Generate and install pkg-config file for slint_cpp

[task-jp]

No description provided.

[ogoffart]

Thanks.
Looks sensible.
I don't know much about pkg-config. How is one supposed to use it? How will it work with the slint-compiler?

[tronical]

I think this may be a generally useful feature to have, but I have concerns similar to Olivier:

• I think the target interface compile features should be propagated (via cflags?) if possible. (For Msvc we’d also need the compile options, but perhaps we don’t need msvc support?)
• It’s unclear to me how other properties like the enabled/disabled feature list becomes accessible to pkg-config users.
• As this may likely break, we should IMO have an automated test that ensures this feature continues to work (build systems are fragile :-)
• If we decide to support pkg-config consuming build systems in Slint, then we need to document how this works, what the limitations are, and how users can correctly invoke the Slint-compiler.

If we can come to an agreement on these points (scope and what is going to be implemented), then I think this makes sense to take in as first PR.

@task-jp could you elaborate on your use-case here? Is cmake not an option in the environment you’d like to use Slint in?